Using a number of complementary anatomical and biochemical techniques, the effects of hyperalgesia and neuropathology on the rat nervous system were studies. Lumbar spinal neurons were retrogradely labeled after injections of retrograde tracer into the caudal midbrain of rats with an inflamed hindpaw. Spinal cord sections from these animals were processed for dynorphin (DYN) immunoreactivity. A larger percentage of projection neurons were contacted by DYN immunoreactive varicosities ipsilateral to the hindpaw. This ipsilateral-contralateral difference was most pronounced in laminase I, a region of spinal cord involved with the processing of nociceptive input. Spinal cord sections of animals with an inflamed hindpaw were processed for glutamic acid decarboxylase (GAD) immunoreactivity. It was observed that the number of neurons with detectable GAD immunoreactivity. It was observed that the number of neurons with detectable GAD immunoreactivity was substantially greater in spinal cord sections ipsilateral to the inflamed hindpaw. Dorsal root ganglia taken from animals with mononeuropathies were analyzed for alteration in mRNA levels for Growth Associated Protein-43 (GAP-43). It was found that mRNA levels increased by day 3 after development of the neuropathy and peaked at day 7.